Currently, cell phones are designed to be increasingly thin with the entire thickness decreased from 12 to 15 mm in the past to 8 to 10 mm, or even less than 6 mm. As the entire thickness of cell phones becomes increasingly small, the space that can be used for design of speaker sound chamber is increasingly small as well. As a moving-coil speaker currently used by cell phones has a relatively large volume (with a thickness between 3 and 5 mm), it is not easy to install or fix the speaker in a limited space of the sound chamber. In addition, a moving-coil speaker has high requirements for the volume of a sound chamber. In the situation that a cell phone has a small space as a sound chamber, the performance of sending out sound is poor, and the quality of sound that is sent out is generally poor. The present invention proposes a design to improve the quality of sound sent out by a cell phone by using a multifunctional battery cover with an ultra-thin piezoelectric ceramic speaker. Therefore, the prior art still needs to be improved and developed.